1. Field of the Invention
The present invention relates to a liquid jetting head which jets or discharges a liquid from a nozzle, and a method of producing the liquid jetting head.
2. Description of the Related Art
As an ink-jet head which jets an ink onto a recording medium, ink-jet heads having various structures have been proposed or brought into practical use. Among these ink-jet heads, an ink-jet head described in U.S. Pat. No. 6,971,738 includes a channel unit in which a plurality of pressure chambers communicating with nozzles respectively is formed, and a piezoelectric actuator which applies pressure to an ink in the pressure chambers, by changing a volume of the pressure chambers.
The piezoelectric actuator of this ink-jet head includes a plurality of piezoelectric sheets made of lead zirconate titanate (PZT) and arranged to cover the pressure chambers, and individual electrodes (drive electrodes) and common electrodes which are arranged alternately between these piezoelectric sheets. The individual electrodes and the common electrodes are formed at an area inside of each of the pressure chambers, along a periphery of each of the pressure chambers, as viewed from a direction orthogonal to a plane of the piezoelectric sheets so as to be ring-shaped.
Moreover, this piezoelectric actuator is capable of easily performing a so-called pulling ejection in which, after an ink in the pressure chambers is drawn in by once increasing a volume of the pressure chambers, a substantial pressure is applied to the ink in the pressure chambers by decreasing the volume of the pressure chambers. In other words, when a driving signal (positive electric potential) is supplied to the individual electrodes while the common electrodes are kept at a ground electric potential, ring-shaped portions of the piezoelectric sheets overlapping with the edge of each of the pressure chambers, which are sandwiched between the individual electrodes and the common electrodes, are contracted in a direction parallel to the plane of the piezoelectric sheets. As a result, the piezoelectric sheets are deformed to project toward a side opposite to the pressure chambers, and the volume inside the pressure chambers is increased, thereby generating a negative pressure wave in the pressure chambers. Furthermore, when the supply of the driving signal to the individual electrodes is stopped at a timing at which the pressure wave is changed to a positive pressure wave in the pressure chambers, the piezoelectric sheets are returned to the original shape to decrease the volume inside the pressure chambers. At this time, the pressure wave generated with the increase in the volume of the pressure chambers and the pressure wave generated upon restoration of the shape in the piezoelectric sheets are combined, thereby applying a substantial pressure to the ink. Therefore, this piezoelectric actuator is capable of efficiently applying the pressure to the ink at a relatively low drive voltage. Moreover, only at a timing of jetting the ink, the driving signal is supplied to the individual electrodes to make an electric field to act in the piezoelectric layers. Therefore, the electric field is not applied in the piezoelectric layers at a timing other than the timing of the ink discharge, and polarization degradation hardly occurs in the piezoelectric layers, thereby enhancing or improving durability of the piezoelectric actuator.